The present invention relates to a spatial light modulator device for the modulation of a light wave field with video hologram information, in particular with discrete complex object light point values of three-dimensional scenes whose object light points are to be reconstructed holographically. The invention is preferably applicable in the context of a holographic reconstruction system that comprises a position controller, an eye finder and an optical wave tracking means which tracks the optical axis of the propagating modulated light wave field to the actual eye position when an observer who watches the holographic information changes their position. Such a holographic reconstruction system has been disclosed for example in WO 2006/119760 A2.
The invention can be used independent of the way in which the holographic information is provided, and it can also be implemented in a system which allows multiple observers to watch holographically reconstructed video scenes simultaneously.
As is generally known, in order to reconstruct three-dimensional scenes by means of video holography, a light wave generator generates a directed light wave field which emits light waves which are capable of generating interference towards the spatial light modulator device. To allow easy addressability of the light modulator device it preferably comprises a regular structure of modulator elements each of which being encoded by a modulator controller with a discrete complex hologram value in accordance with the spatial arrangement of object light points in a video scene to be reconstructed.
In the present document, ‘encoding’ shall be understood to be the discrete adjustment of the optical transmittance of modulator cells of the light modulator device. As a result of this encoding, the modulator cells modulate incident light wave portions of the light wave field which is capable of generating interference such that multiple emitted light wave portions reconstruct by way of constructive or destructive interference the plurality of object light points of a scene that is described by a video signal in the space on the optical path downstream of the spatial light modulator means, seen in the direction of light propagation.
In the context of the present invention, discrete complex hologram values carry holographic information for discrete encoding of a modulation array with a video hologram. The modulator controller writes to each modulator element components of the holographic code, such as a real part and an imaginary part in terms of complex numbers, in order to affect amplitude and/or phase of the light transfer function of each modulator element.
Conventional light modulator devices usually serve either as amplitude-only modulators or as phase-only modulators, thus only influencing the light waves with a single real value. This means that these modulators locally change either the amplitude only or the phase information only through their cell encoding.
In video holography, light modulator devices must be capable of working in real-time, and they must be able to produce full-colour reconstructions in a large reconstruction space.
Each light modulator device comprises at least one modulation array with regularly arranged modulator elements, where each modulator element comprises a number of modulator cells. A modulation array is typically realised by a spatial light modulator (SLM). Such a spatial light modulator has individual modulator cells, commonly also known as pixels.
According to the principle of hologram reconstruction, the modulator controller simultaneously computes discrete complex hologram values for all modulator elements which are involved in the holographic reconstruction of an object light point based on the corresponding discrete complex value of the object light point of the scene. Corresponding code value components are generated for each discrete complex hologram value prior to the encoding process. The code value components for each modulator element are computed and mutually adjusted such that all modulator cells of each modulator element interact such that the complex local light modulation which is expected from the modulator element is actually achieved. The complex object light point values are computed by the modulator controller prior to the encoding for example based on a video signal with depth information.
Document U.S. Pat. No. 5,416,618, for example, discloses a light modulator device which comprises a combination of multiple stacked spatial light modulator arrays. For example, one light modulator array with amplitude light modulator cells and one with phase light modulator cells, or two arrays with light modulator cells of the same kind, are stacked in the direction of light propagation. This stacking generates the modulator elements which comprise multiple single cells and which modulate a light wave field with complex hologram values. It is a disadvantage here, however, that when the light modulator arrays are adjoined, considerable adjustment efforts must be made in order to realise exact congruence of the cell structures.
This disadvantage does not occur though where a complex hologram value is realised by a group of multiple modulator cells of one modulation array, in particular when the multiple modulator cells are arranged side by side in respect of the direction of light propagation.
The light modulator device according to the present application therefore comprises at least one modulation array with regularly arranged modulator cells which can be encoded discretely but which are combined to form joint modulation elements as regards their optical effect and their electrical addressing. The modulation elements of the light modulator device are arranged in an array and modulate in a spatially structured way the light waves which are capable of generating interference of the propagating light wave field. This means that each modulation element only changes a light wave portion of the propagating light wave field which actually hits the modulation array in accordance with the actual hologram value for the modulation element. For this, the modulator controller provides for each individual modulator cell a separate value component of the complex object scanning value which is assigned to the modulation element.
Each modulation element thus comprises a combination of modulator cells, where the modulator cells can be realised in the form of phase-modulating light modulator cells or amplitude-modulating light modulator cells. This means that, depending on the design and local arrangement of the modulator cells, each modulation element can modulate a light wave portion of the impingent propagating light wave field as regards its wave phase with one modulator cell and its wave amplitude with the other modulator cell, or as regards its wave phase or wave amplitude only with all modulator cells.
The general principle of the spatial light modulation as described above with modulation elements which provide phase modulation only, which can be addressed irrespective of each other with different value components per modulation element, e.g. according to the two-phase encoding method, has already been described by the applicant in document WO 2007/082707 A1.
The above-mentioned publication shows a preferred way of encoding a spatial light modulator device with multiple phase values. A complex object scanning value is represented by a sum of two phase components with same absolute amplitude value and different phase values and encoded to two adjacent phase-modulating light modulator cells of the same modulation array. This means that each complex object scanning value with the phase y and amplitude a ranging between 0 and 1 is thus composed of the sum of two complex phase components with the same amplitude value and the phase values ψ±acos a. It is also mentioned in the international patent publication that the number of phase modulator cells that constitute a modulation element is not necessarily limited to two.
A spatial light modulator device which provides phase modulation only has great advantages over light wave modulation with modulator cells for amplitude modulation. A light modulator device with two-phase encoding shows the reconstruction at greater brightness, because the modulator cells realise maximum light transmittance with each phase setting. Another advantage of the two-phase encoding method is that it provides a more favourable wavelength dependence during reconstruction, which allows colour video scenes to be reconstructed at high quality.
The mentioned two-phase encoding method is meant to achieve the situation that those light wave portions of the light wave field which is capable of generating interference which are modulated by adjoined modulator cells of a modulation element show the same optical interference effect as those light wave portions which are modulated by such a single modulator cell that is simultaneously addressed with all phase components of a complex object scanning value.
However, this is difficult to be realised because the modulator cells which are combined to form a modulation element lie side by side in the modulation array, thus having a spatial offset and showing differences in the length of the optical path, also known as retardation, with a magnitude that depends on the kind of hologram, on the eye position of an observer who sees reconstructed object light points, and—for example with Fourier holograms—on the position of the object light point which is to be reconstructed by these modulator cells. This offset of the modulator cells effects phase differences among the modulator cells of the modulation elements, said phase differences depending on the position of an observer eye and from the desired angular position of the reconstructed object point in respect of the optical axis of the system (depending on the kind of hologram) and impairing the quality of the reconstruction of the video scene, so that they require correction of each modulation element. In a holographic reconstruction system with position controller and eye finder, which, as described above, track the propagating modulated wave field optically upon a change in an observer position, it is also very advantageous to have a tolerance in the observer position of a few millimeters around the eye position which has been detected by the eye finder. The occurring differences in the optical path lengths would substantially restrict this slight freedom of movement of an observer in front of the holographic reconstruction system while watching a holographic reconstruction.
A solution to this problem has been suggested in the international patent application WO 2008/132206 A1 titled “Light modulator for representing complex-valued information”. According to this solution, a structured retardation layer made of a birefringent material is arranged in the optical path of the modulation array, i.e. upstream and/or downstream of the modulation array and preferably in close contact with the modulator cells, said layer effecting an angular-position-specific adaptation of the optical path length of the emitted modulated light wave portion to the lengths of the optical paths through the other cells of the modulation element at least for one modulator cell of each modulation element. The layer thickness of this retardation layer is chosen such that the retardation layer counteracts the angular-position-specific change in the optical path among the modulator cells of each modulation element by way of changing the optical path lengths and compensates them at least partly. This solution has the disadvantage that it requires a very finely structured but at the same time rather thick layer.
The device disclosed in the unexamined application DE 2 058 418 titled “Device for determining the position of an object in an arbitrary section of a pencil of rays” takes advantage inter al. of a point light source and a Savart plate. That Savart plate comprises two serially arranged birefringent uniaxial plate-shaped crystals which are arranged such that their main sections are turned to a perpendicular situation, where the angle of the optical axis and the crystal surface is the same for both crystals. The document teaches that the Savart plate divides the pencils of rays which are originally emitted by an point light source and which are incident on its entry side into two linear-polarised partial pencils which show mutually perpendicular polarisation and appear to origin in two linear-polarised virtual light sources which lie on the entry side in a plane in symmetry with the original light source. The document further teaches that there is no difference in optical path length among partial pencils of rays which show mutually perpendicular polarisation in each point of the plane to which the virtual light sources are arranged in mirror-symmetrical arrangement. In any other points, there are differences in optical path length among the partial pencils.
In the present invention, the term ‘Savart plate’ is used to designate any at least one single birefringent plate-shaped uniaxial crystal. Further, the birefringent material shall not be limited to conventional crystals such as quartz or calcite, but can also be generated for example by an oriented polymer and/or a suitable polymer layer or suitable film.
In the context of the present invention it is immaterial how the modulator cells are actually designed. It is for example possible to use a modulation array of liquid crystal cells or of electrowetting cells. The modulator cells can be of such nature that the modulation array modulates the light waves of the wave field during their passage or when being reflected.
An alternative to a spatial light modulator device with phase-modulating light modulator cells can be a modulation array which exclusively comprises amplitude-modulating light modulator cells, where each modulation element is composed of multiple amplitude-modulating light modulator cells. An encoding method for a light modulator device which comprises two amplitude-modulating light modulator cells, namely one for the real part and one for the imaginary part of a complex number, is known as bias encoding. Another encoding method for a light modulator device which comprises three amplitude-modulating light modulator cells is known as Burckhardt encoding.
A phase error which substantially impairs the quality of the reconstruction always occurs in a complex modulation element with multiple adjoining modulator cells as a consequence of the angular-position-specific difference in the optical path lengths among the individual modulator cells, irrespective of the nature of the modulator cells of the modulation array.